The present invention relates generally to air filtering or cleaning systems and, more particularly, to an apparatus for and method of testing the integrity of individual filters in a multistage filtering system embodying a plurality of filters.
In air cleaning and filtering systems, and particularly nuclear air cleaning facilities, it is important that all filters employed in such systems be absolutely leak-tight to preclude contamination of the effluent ultimately discharged into the atmosphere. Typically, these systems include two or more filter stages in series located in a common housing, each stage comprised of a bank or plurality of filters arranged horizontally and/or vertically in a common plane. In order to check the integrity (leak-tightness) of these filters to achieve and maintain the high performance levels required of an operative, high efficiency air cleaning system, the in-place testing of the system is necessary.
The usual in-place testing of a filtering system includes introducing a trace or test agent in the form of an aerosol upstream of the filtering system, the particles of which are dispersed uniformly over the upstream face area of the filter stage being tested. An upstream sampling device is positioned in close proximity to the upstream face of the filter to determine the trace agent concentration admitted to the filter. Any particles which penetrate the body of the filter or leak through the gasket or filter frames, etc., pass into the downstream region of the filter stage where it is thorougly mixed with the filtered air. A sample of this filtered mixture is withdrawn to determine the trace agent concentration therein for comparison with the upstream trace agent concentration to calculate the extent of leakage, if any, of the filter stage. In order to assure adequate dispersion of the trace agent within the airstream to obtain a representative trace agent concentration in the above discribed procedure, the trace agent must be introduced into the airstream a distance equaling at least 10 duct diameters upstream of the filter stage being tested. Likewise, the representative sample of trace agent concentration penetrating the filter stage must be withdrawn a substantial distance downstream therefrom.
As heretofore mentioned, a nuclear air cleaning system usually contains two or more filter stages in series in the same container. This physical arrangement, which is dictated by space limitations and cost considerations and cannot accommodate the space requirements of the procedure described above, coupled with the laminar air flow between the stages poses special problems in an in-place testing procedure. Specifically, it is difficult to obtain uniform distribution of the trace agent within the airstream between two adjacent filter stages in order to obtain an adequate dispersion for a single point representative penetration sample taken downstream of the filter stage being tested.
Various attempts have been made to solve this problem, including the use of temporary bypass ducts, appropriate closure valves or covers, and mechanical or electrical devices to isolate the areas between adjacent filter stages for injecting the trace agent therein and/or discharging the filtered air and trace agent mixture therefrom for subsequent downstream sampling. Where filter stage leakage is detected, it is sometimes necessary for personnel to probe scan individual filters to identify the defective filter. Not only is this testing technique costly in providing the necessary hardware and labor but, more importantly, exposes personnel to radioactive or other toxic environments. Indeed, this procedure can not be employed at all in those hostile environments where radiation reaches such magnitudes that personnel access is prohibited.
Other efforts to overcome the above noted disadvantages are disclosed in U.S. Pat. No. 4,055,075 to Allan et al and U.S. Pat. No. 4,324,568 to Wilcox et al. for example. U.S. Pat. No. 4,055,075 discloses a method and apparatus for the leak testing of filters including means for imparting a like electrical charge to the trace agent particles so that the particles repel each other and thereby disperse within the airstream. A plurality of oppositely charged collecting plates are mounted adjacent the upstream face of each filter to accumulate particles and thereby determine the upstream particle concentration. Also, an oppositely charged collecting plate is mounted downstream of the filter to determine the extent of particle leakage past the filter. The disadvantages residing in this arrangement are the extra electrical equipment and components required as well as the deterioration and possible failure thereof requiring some maintenance and possible replacement. Also, any shorts in the system pose a safety hazard in certain volatile environments. Moreover, the positioning of the particle collecting plates would interfere with the otherwise uniform flow through the filters.
In another embodiment disclosed in the subject patent, a collapsible panel is employed to condense and impart turbulence to the airstream in order to disperse the particles therewithin. Such an arrangement involves actuating mechanisms and movable panels vulnerable to wear and resulting failure with consequent contact maintenance.
U.S. Pat. No. 4,324,568 discloses another approach for leak testing filters within restricted space requirements and utilizes a funnel-shaped member moveable into an operative position for condensing and mixing the airstream and particles and then dispersing the same over the frontal area of the adjacent filter. Again, this structural arrangement requires costly hardware and moveable parts vulnerable to wear and possible failure with resultant contact maintenance.